1. Field of the Invention
The present invention relates to a synchronous follow-up circuit for following up the synchronization of a received signal in a receiver or the like for receiving a signal spread with a spread code to acquire the synchronization of the received signal.
2. Description of the Background Art
Conventionally, some of the receivers or the like for receiving a signal spread with a spread code are adapted to roughly acquire the synchronization of the received signal through its synchronization acquisition circuit, and follow up the synchronization of the received signal through its synchronous follow-up circuit.
For example, a symbol synchronous circuit disclosed in Japanese patent laid-open publication No. 204932/1994 (patent document 1) generates a timing pulse in a symbol timing generator, shifts the phase of the timing pulse to generate a plurality of timing signals, compares those timing signals with a converting point pulse train, and selects an optimal timing signal to thereby accomplish the following-up of the synchronization.
Further, another type of synchronous follow-up circuit is disclosed in Japanese patent laid-open publication No. 18446/1997 (patent document 2). That synchronous follow-up circuit is adapted to receive by a symbol correlator signals having the respective phases thereof ordinary, advanced resultant from a multiplication with an Early PN (Pseudo Noise) code and retarded resultant from a multiplication with a Late PN code to obtain correlation values, and to compare on the basis of their mean value the difference or gradient with a threshold value to determine the synchronization, retardation and advance of the phase to follow up the synchronization.
In a symbol synchronous circuit disclosed in the above patent document 1, when instantaneous noise is included in a received signal, it also effects on a symbol correlation power so that an erroneous symbol phase is detected to cause out-of-synchronization. Particularly, in the case of complementary code keying (CCK) modulation where a spread code is extracted from a received signal to despread, when a spread code is erroneously extracted, despreading (symbol correlation) is performed based on that incorrect spread code, which leads to detection of an incorrect symbol phase, which in turn causes out-of-synchronization.
Further, in a synchronous follow-up circuit disclosed in above patent document 2, in order to avoid the out-of-synchronization caused by noise, there is introduced averaging of power values, while in such a case the speed following up the synchronization lowers due to operation for averaging, which may cause delay in detection timing.